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The knee is a complex joint involve different structures such as bones, tendons, muscles and ligaments. They all work together to maintain usual knee function and also provides stability to the knee.

A well-functioning healthy knee is essential for our mobility in day to day life and to participate in various sports and other activities. Understanding the anatomy of the knee and how different parts of the knee work together will help us to know how various knee injuries and disease condition occur and can affect the structure and function of the knee. Moreover, it enhances our ability to discuss with the physician and choose the right treatment procedure for a knee problem.

Bones

The Knee is a hinge joint where the thigh bone (femur) moves over the shinbone (tibia). The end of the femur that articulates with the tibia is shaped like two round knobs. These round knobs are called femoral condyles. These two condyles rest on the flat surface of the tibia which is also known as tibial plateau. The surface of the tibia plateau that articulates with the femoral condyle farthest away from the other knee (i.e. on the lateral side) is called as lateral tibial plateau and the other half that articulates with the other condyle closest from the other knee (i.e. on the medial side) is called as medial tibial plateau.

The two femoral condyles form a groove on the front (anterior) side of the knee called patellofemoral groove. A small bone sits in this groove and forms the knee cap, which is known as patella. It acts as a shield and protects the knee joint when you directly fall on the knee.

A fourth bone called fibula is a bone of the lower leg. This forms a small joint with the tibia. This joint has very little movement and is not really the part of the main joint of the knee.

Articular Cartilage and Menisci

Movement at the knee joint involves these bony surfaces to move against each other. This will result in friction between the articulating surfaces of the bones and result in damage to the articulating surfaces. Hence, to reduce this friction all the articulating surfaces of the bones involved in movement are covered with a white, shiny, slippery and about one- quarter-of-an-inch thick layer, called articular cartilage. Thus, the articulating surface of the femoral condyles, tibial plateau and the back of the patella are covered with an articular cartilage. Articular cartilage provides them with an extremely smooth surface that facilitates movement.

To further reduce friction between the articulating surfaces of the bones, the knee joint is enclosed in a ligament capsule, which is lined with a synovial membrane on the inner side that produces a thick clear fluid called synovial fluid. This fluid lubricates and nourishes the cartilage and bones inside the joint capsule. Thus, in a healthy knee friction is almost zero.

Within the knee joint, between the femur and tibia, there are also two C shaped cartilaginous structures called menisci. Menisci convert the flat tibial plateau into a shallow socket and help to give stability to the knee. The menisci also spread the weight on the femur across the whole surface of tibial plateau and prevent the weight to concentrate onto a small area which may damage the articular cartilage. Thus, menisci help in load bearing. The menisci also act as a cushion between the femur and tibia, and absorb the shock produced by activities such as walking, running and jumping.

Ligaments

Ligaments of knee are the most important structures that stabilize the knee joint. These are tough bands of tissue that connect a bone to another bone. There are two important groups of ligaments that hold the bones of the knee joint together as ropes. These are:

Collateral ligaments- Collateral ligaments are present on either side of the knee. They prevent the knee from moving too far in side to side motion. Collateral ligament on the medial side is called as medial collateral ligament (MCL) and collateral ligament on the lateral side is called as lateral collateral ligament (LCL).

Cruciate ligaments- This group of ligaments are present inside the knee joint and control the back and forth motion of the knee. Cruciate ligament in the front of the knee is called anterior cruciate ligament (ACL), whereas, in the back of the knee is called as posterior cruciate ligament (PCL). ACL and PCL cross each other within the knee joint.

Muscles

There are two major muscle groups, quadriceps and hamstrings that allow movement of the knee joint. The quadriceps muscles are located in the front of the thigh. These muscles function to extend the knee, which in turn straightens the leg. The hamstrings are found in the back of the thigh and these musclesactively bend(flex) the knee.

Tendons

Tendons are structures that attach muscles to the bone. The four quadriceps muscles of the knee meet just above the patella and attach to the patella through a tendon called quadriceps tendon. The patella further attaches to the tibia through a tendon called patella tendon. Working together, the quadriceps muscles, quadriceps tendon and patellar tendon straighten the knee.Similarly, hamstring muscles on the back of the leg connect around the knee joint by three hamstring tendons.